1. Field of the Invention:
The present invention relates generally to a variable band pass filter circuit, and more particularly is directed to a variable band pass filter circuit suitable for use with an intermediate frequency amplifier circuit of a radio receiver.
2. Description of the Prior Art:
When an AM broadcasting wave is received in an area where the AM broadcasting wave is feeble, there is a possibility that the S/N ratio will deteriorate to an undesirable extent. Accordingly, recently, in the art, in order to improve the S/N ratio, the pass band range of an intermediate frequency amplifier circuit has been made variable or the pass band range thereof has been narrowed in the area where the broadcasting wave is feeble, so as to eliminate jamming waves.
Variable band pass filter circuits whose pass band range can be as desired as well as various intermediate frequency amplifier circuits have been proposed in the art. For example, an LC-type filter circuit which is formed of a coil forming an intermediate frequency transformer. A capacitor is connected in parallel to the coil, and the number of turns of the coil is varied by a switch to vary the pass band of the filter. However, if the prior art filter circuit is formed as a multituning type, a switch for each filter circuit, is required which results in a circuit construction of such a multi-tuning type filter circuit which is complicated and, it is difficult to maintain the desired selectivity.
In another prior art filter circuit in which a number of ceramic filters are used, two capacitors are connected through a switch in series or parallel between adjacent ceramic filters so as to vary the pass band of the filter circuit. Ceramic filters are described in U.S. Patents No. 2,708,244, which issued May 10, 1955, U.S. Pat. No. 2,943,279 which issued June 28, 1960 and U.S. Pat. No. 2,969,512 which issued Jan. 24, 1961. With this prior art filter circuit, its center frequency f.sub.o (for example, 455 k Hz) is deviated or shifted by changing the capacitors. Also ceramic filters are rich in spurious component which does not improve the S/N ratio. In fact, if the constant of the ceramic filters is previously selected to be so as proper to make its resonance frequency somewhat higher than the center frequency f.sub.o, the deviation or shift of the center frequency f.sub.o caused by changing the switch can be compensated. However, such ceramic filters are difficult to obtained and used in practice.
In any case, the prior art filter circuit is complicated and its center frequency f.sub.o will be shifted.
At present, there has been proposed no variable band pass filter which is free from the above defects.